The biosynthesis of catecholamines is dependent upon the hydroxylase cofactor tetrahydrobiopterin (BH4). Since the concentration of BH4 in brain tissue is thought to be less than saturating for tyrosine hydroxylase and that the level of this compound in CSF may be reflective of the concentration in brain, it was of value to measure the BH4 content in CSF of selected subjects. This study was undertaken to detect and monitor CSF hydroxylase cofactor activity using a phenylalanine hydroxylase assay. Our studies indicate hydroxylase cofactor activity declines with age in both normal and Parkinsonian patients. No significant alteration as found in CSF cofactor levels of schizophrenic or affective disorder patients. We have reported that patients with certain neurological diseases, most notably Parkinson's disease have reduced levels of cofactor in their CSF. In addition, certain patients with generalized dystonia have relatively low CSF cofactor content.